1. Field of the Invention This invention relates generally to tunable diode lasers, and particularly, to an improved tuning system which avoids tuning discontinuities by maintaining a constant integral number of half wavelengths in the tuning cavity over the entire tuning range.
2. Description of the Prior Art Tunable semiconductor diode lasers, which provide an extremely useful optical tool, are handicapped by the fact that the tuning mechanisms do not maintain a constant number of half wavelengths within the cavity. The typical tunable diode laser has one facet having an antireflection coating through which a selected wavelength is fed back into the laser to sustain oscillation at the selected wavelength. Quite commonly, the desired wavelength is selected by means of a diffraction grating with a rotatable mirror which selects a desired wavelength from the beam diffracted by the grating. The variation in angle of the mirror is effective to select the desired wavelength, which is diffracted by the grating at the angle represented by the mirror position. While this approach provides a convenient means for tuning the operating wavelength of the laser, it has been found that mirror rotation alone does not provide a smooth tuning action. This is because the simple rotation of the mirror to select the energy emitted from the diffraction grating at the angle of the desired wavelength does not maintain the length of the tuning cavity at an integral number of half wavelengths. As the wavelength is varied and the number of waves in the cavity varies, the laser output exhibits discontinuities such as large changes in output power.
The basic principles of the operation of the tunable laser utilizing a variable length external cavity in conjunction with a diffraction grating and a rotatable mirror are set forth in the publication, "Spectrally Narrow Pulse Dye Laser Without Beam Expander", by Michael G. Littman and Harold J. Metcalf, Applied Optics, vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978 Although the article describes a system which uses a dye laser, the diode laser is easily substituted. The system utilizes a diffraction grating which is filled with an incident collimated laser beam by using the grating at a grazing angle. The diffracted beam at the angle normal to the mirror is reflected back onto the grating and from there, back into the lasing cavity, where it serves to determine the operating wavelength of the system. Rotation of the mirror to select the wave diffracted allows the system to be tuned to a desired output wavelength.
It was later recognized that simple rotation of the mirror did not provide a continuous single mode scan over a range of wavelengths. The publication, "Novel Geometry for Single-Mode Scanning of Tunable Lasers" by Michael G. Littman and Karen Liu, Optics Letters, Vol. 6, No. 3, pages 117, 118, March, 1981 describes a tunable cavity in which the mirror is translated axially, as well as rotated, to change the cavity length as well as the angle of the diffracted beam returned to the laser. Although the authors state that the pivot point selected by their method provides exact tracking for all accessible wavelengths, this is, in fact, true only for the case where there are no dispersive elements on the cavity since the changes in optical length due to the effects of dispersion are not considered. Further information of a general nature is available in "Introduction to Optical Electronics" by Amnon Yariu, 1976, published by Holt, Rinehart and Wilson; and "Optics" by Eugene Hecht, 1987, published by Addison-Wesley Publishing Co.
The shortcomings of tuning systems in which the mirror was rotated only was further developed in the publication "Synchronous Cavity Mode and Feedback Wavelength Scanning in Dye Laser Oscillators with Gratings" by Harold J. Metcalf and Patrick McNicholl, Applied Optics, Vol. 245, No. 17, pages 2757-2761, Sept. 1, 1985. The geometry described in this publication relates to positioning the point of rotation (pivot point) of the mirror at the intersection of the planes of the surface elements. The article suggests that for oscillators with mirrors as both end elements a possibly useful displaced configuration will also be synchronous. However, the displaced configurations will, as above, be synchronous only in the absence of dispersive elements in the cavity.
A further development, set forth in "External-Cavity Diode Laser Using a Grazing-Incidence Diffraction Grating", by K. C. Harvey and C. J. Myatt, Optics Letters, Vol. 16, No. 12, pages 910-912, Jun. 15, 1991, describes a tunable cavity system utilizing a diode laser in which the diode laser has a highly reflective rear facet and an anti-reflection coated output facet with an output window. The output beam is collimated by a lens and illuminates a diffraction grating at a grazing angle. The first order of diffraction of the grating is incident on the mirror, which reflects it back onto the grating, where the first order of diffraction passes back into the diode laser. The output of the system is the zeroth-order reflection from the grating. In this system, no mention is made of coordinated rotation and lineal translation of the mirror.
Various mechanical arrangements for movement of the mirror have been devised to introduce simultaneous rotation and longitudinal translation in attempts to maintain the physical length of the internal cavity at a constant number of half wavelengths. One such a system is shown in U.S. Pat. No. 5,058,124 to Cameron et al.
Despite efforts to devise suitable mechanical arrangements for simultaneous rotation and linear translation of the mirror, the prior art systems either require complex adjustment or do not effectively provide the required constant number of half wavelengths within the tuning cavity. In the case of a cavity containing dispersive elements, even the dispersion caused by air can be a significant factor.